1. Field of the Invention
The present invention relates to the field of gas chromatography. More particularly, the present invention relates to multi-dimensional gas chromatography and interfaces therefor.
2. Description of the Prior Art
Multidimensional gas chromatography is an analytical technique which further enhances the resolving power of single column gas chromatography. The term dimension, in the context of this technique, is defined as packed or capillary columns in a multiple column system, each column having differing selectivities based on their stationary phases. In general, two columns are connected in a manner that allows chromatographic separation on a primary column, followed by the introduction of regions of eluent or "heart cuts" from that column into a secondary column. The system which provides for the collection and manipulation of the heart cuts is known as the "system interface".
Three general design approaches for system interfaces have previously been employed in multi-dimensional gas chromatography. The use of rotary valves was developed as a column coupling mechanism as early as 1958 to separate C.sub.5 through C.sub.7 hydrocarbons on packed columns (see K. Himberg et al., Organohalogen Compd., vol. 4, Misc. Contrl., 1989, pp. 183-186). Rotary valves are simple to use, however, two problems have been encountered with their use in system interfaces. First, switching valves have a large thermal mass. When temperature gradients are applied during gas chromatographic analysis, the switching valve temperature does not rise at the same rate as the rest of the chromatographic column (thermal lag). The resulting "cold spot" causes broadening of peaks and inconsistent retention times. Second, switching valves may have exposed metal surfaces that catalyze the reactive degradation of unstable analytes.
The limitations of existing rotary valves led to the development of the "Dean's switch" in 1968 (see B. Gordon et al., J. Chromatogr. Sci., vol. 23, No. 1, 1985, pp. 1-10). The Dean's switch was the first attempt to enable live, valveless, multicolumn gas chromatography by placing a pneumatic switch between two columns in a series. This switching is possible only through the precise balance of carrier gas pressures at the inlet and at the column connection.
A third distinct approach to performing multidimensional chromatography has been the use of controlled temperature regions to generate "wave fronts" of sample on directly linked columns (thermal modulation). In this approach, units of column are treated with a resistive coating which is heated by applying a potential across it. The entire sample is therefore chromatographed on both columns successively (see U.S. Pat. No. 5,135,549 to Phillips et al. (1992)).
The Dean's switch, as used in present commercially available instruments or retrofit devices capable of performing multidimensional gas chromatography, is a pneumatically controlled interface system which uses slight differences in inlet and outlet pressure to cut windows of column eluent from a primary column to a secondary column. Essential to the operation of the Dean's switch is the use of a column coupling piece which directs flow from a primary column to either a monitor detector or an analytical column. Prior to the introduction of the "live T" (see G. Schomburg et al, Chromatographia, vol. 16, 1982, pp. 87-92), Dean's switch coupling components exhibited catalytic degradation of sample components. The predominant system used today, the Siemens Sinchromat 2 (Siemens AG, Karlsruhe, Germany), incorporates Schromburg's "live T". This coupling component is a six-port union which includes a platinum/iridium insert. Two ports serve to connect columns in a series, two ports connect to carrier gas flows, and two ports connect to detectors through flow restrictors. Solenoid valves, needle valves, and restrictors outside the direct column flow path are precisely adjusted and switched to cut eluent from the first column onto the second. Difficulties encountered in Dean's switch operation are generally due to the nature of trying to control minute pressure differences while varying column temperature. Column flow rate is inversely proportional to temperature, and pressure is directly proportional to temperature. To resolve these difficulties, Abbott attempted to direct pre- and post-column flows (peripheral to the coupling piece) with mass flow controllers (see D. J. Abbott, J. High Resolut. Chromatogr., Chromatogr, Commun., vol. 7, No. 10, 1984, p. 577). Because of slower equilibrium times associated with the controllers, narrow cuts from the primary column could not be obtained.
Single or multiple stage thermal modulation is a process that involves the cooling and heating of regions of chromatography columns coupled in series. Cooling of a discrete region of a column causes sample to be focused onto a narrow band. A rapid increase in temperature drives that band off as a wave. This accumulation, transfer, and release of sample between temperature controlled regions of a column allows small portions of a complete sample to be sequentially introduced onto the second column in series. Multiple stage thermal modulation involves creation of several thermally controlled regions of column, normally near both the inlet and the outlet of each column. Heating of regions of column has been accomplished by treating them with a resistive coating and applying a voltage across the resistance (see U.S. Pat. No. 5,135,549 to Phillips, above).
The most pronounced advantage of thermal modulation is the ability to chromatograph an entire sample on both a primary and a secondary column. Disadvantages include the fact that detectors which are easily contaminated suffer insult from everything introduced on column. Also, because thermal modulation alone doesn't allow coupling of mismatched column diameters or the flexibility to make multiple injections, trace enrichment is not possible on the system design. Elaborate temperature programming is necessary to manage narrow bands of sample. Finally, an instrument modified for thermal modulation is a dedicated instrument which does not allow the flexibility of using either column independently.
Mechanical valve coupling of multiple columns is the most direct and simple means of multidimensional gas chromatography (See Gordon, above). Column flows are set as they would be in one dimensional analysis. Because sample flows are directed through low voltage paths in rotary valves, flexibility in design and use are maintained. Non-dedicated instruments can be maintained to allow an operator the option of either one-dimensional analysis or two-dimensional analysis without major reassembly of the instrument.
Rotary valve system interfaces initially suffered from two primary drawbacks. The valves exhibit a large thermal mass due to their bulky nature, and, prior to the advent of micro valves, large metal surface areas within the valves served as sites of activity (acidity, adsorption, or poor efficiency). Minimized activity and reduction of thermal lag in rotary valves was accomplished by using a valve which was independently heated, and had flow paths equivalent in diameter to its attached capillary column.
Several rotary valve system interface design variations have been used to accomplish the task of GC column coupling. The most common approach is the use of a single six port bimodal valve which mimics the path configuration of the "live T". However, with a rotary valve, no pressure balancing is necessary with columns of identical diameter and length. Wong employed a combination of four port and six port bimodal rotary valves in a system interface in the multidimensional analysis of d-arabinitol in serum (see B. Wond, J. Chromatrogr., vol. 495, 1989, pp. 21-30).
It is desirable in many analyses to interface capillary GC columns having substantially different bore sizes. This enables large volume samples to be injected and separated with ultimate high component resolution and sensitivity. Large injection volumes of hundreds of microliters can routinely be made on wide bore capillary columns (typically about 0.53 mm I.D.). In contrast, even with the new pressure pulsed injection techniques being used today, injection volumes of only 5 to 10 microliter can be made onto the narrow bore GC columns (typically from 0.18 to 0.25 mm internal diameter). In the case of a GC configured to a mass spectrometer, column flow rates of only about 1 ml/min. of carrier gas can be accommodated in the vacuum chamber of the detector. This limits the GC selection to only narrow bore capillary. Therefore, only small injection volumes can be made onto the GC/MS making the detection limits of a given method a function of only the sample preparation, and the instrument detection limit. Existing system interfaces do not have the capability to interface such wide bore/narrow bore GC systems effectively.
The present invention overcomes several problems encountered in the prior art including cold spots due to the large thermal mass of rotary valves, the requirement to dedicate an instrument solely to two-dimensional analysis, and the lack of isolated cold traps, while providing a two-dimensional wide bore-narrow bore chromatography system capable of convenient use. The inventive system provides a dedicated oven for maintaining the rotary valves at an elevated temperature, thereby eliminating thermal lag in temperature programmed chromatographic analysis. Column flow adjustments are made at the column inlets eliminating the need for delicate pressure adjustment as required in present Dean's switch systems. The provision in the present inventive system interface of a heartcut trap isolated from any carrier gas flows allows for easy maintenance of cold trap equilibrium and minimized sample breakthrough.
The advantages of the present invention can be illustrated by a comparison with the Wong device (see above). The Wong system interface features mechanical valve heads contained within the same oven as the chromatography column. As temperature programs are executed, valve temperatures lag behind column temperatures because of the large thermal mass of the valve bodies. Cold spots are created within the valves which disrupt chromatography. A four port valve serves to direct primary column flow to either a monitor/detector or a cold trap (liquid CO.sub.2). The second valve, a six port valve, serves to direct flow from the cold trap to the analytical column/detector. Because of the narrow internal diameter of the paths in the valves (0.25 mm), and because any flow from the primary column must flow through both valves, the use of two columns of drastically different internal diameters is not possible without changing the configuration or balancing flows with restrictors. If multiple analytes are to be trapped, possible losses may occur during the chromatographic run as carrier gas is constantly flowing through the cold trap. In comparison, the inventive system and system interface employs a bimodal six port valve having an internal diameter comparable to the primary column from which it directs flow. All separation/cleanup and trapping of sample components is accomplished while avoiding the directing of flow through the second valve (a small internal diameter bimodal four port valve). This enables the use of columns with very different internal diameters to be coupled without adjusting flows or pressures to compensate. The second valve of the present invention is comparable in inside diameter to the small bore analytical column for which it directs flow. A second carrier gas inlet source allows either direct access to the analytical column or to the cold trap. Valve bodies and cold trap are kept in a separate oven at elevated temperatures, eliminating thermal lag effects. The inventive flow configuration allows any trapped analytes to be completely isolated from carrier gas flow until the desired analysis time.
Column flow in a gas chromatographic system is determined by several variables: column length, internal diameter (including any additional restriction within the flow path), column head pressure, stationary phase and carrier composition, and thermal expansion during temperature programming. In the inventive configuration, each column is completely isolated from its series counterpart during all portions of an analysis. In addition, no restriction is introduced into the path of either valve which would reduce the flow during any portion of an analysis, even when valves are rotated for peak capture, etc. This allows flows for diverse capacity columns to be set and maintained independently without affecting the flow of the other column. A further consideration is that, using valve internal diameters which are too large increases inefficiently swept flow system volumes (dead volumes). As pointed out by Jennings (see W. Jennings, J. Chromatogr. Sci., vol. 22, 1984, pp. 129-135) minimizing internal diameter of mechanical valve paths diminishes degradation of sample components within the valves. In the inventive design, each valve path is minimized to accommodate its corresponding column without unduly restricting flow to the complimentary column. In contrast to the inventive design, a prior art design such as that of Wong, above, which limits valve diameter during all switching steps, will disrupt system flow, necessitating the coupling of identical or nearly identical columns through the minimized diameter valve flow paths.
The present invention, then, provides for optimum sample flows in a multidimensional gas GC system employing significantly differing column bore widths, consistent with economical system interface design, while avoiding the difficult pressure balancing procedures or sample deterioration of prior art systems.